docs(readme): web-first framing + standalone HTML build path (#90)

## Summary

Brings the README in line with the v5 codebase and the [`modernization-roadmap.md`](.claude/rules/modernization-roadmap.md) web-only direction. The previous README still framed ExifCleaner as a desktop app first — "Desktop app + browser webapp" tagline, a "Download and Install" section linking to upstream `szTheory/exifcleaner` .dmg / .exe / .AppImage from v3.6.0 (May 2021, pre-Phase-D).

## Changes

- **Tagline** → "Runs entirely in your browser — no uploads, no server."
- **Features list** → drops the cross-platform line; leads with "Browser-based: install as a PWA or run from a single self-contained HTML file." Also corrects the privacy-controls line — Phase F #83 enforces epoch timestamps, there is no "preserve timestamps" setting (per [`privacy-invariants.md`](.claude/rules/privacy-invariants.md) §6).
- **Remove "Download and Install" entirely** — the upstream desktop links pointed at the legacy v3.6.0 binaries (last released May 2021); they don't reflect the v5 codebase, and sending users there is misleading.
- **"Running the web app locally"** gets a new **Option 1: Single-file HTML** via `yarn build:web:standalone` (lands in [docs/standalone-html.md](docs/standalone-html.md), shipped in #89). Docker / Node dev server / build+preview shift to Options 2–4.
- **Drop the screenshot** — `static/screenshot.png` pre-dated the BEFORE/AFTER/RESULT columns (#88) and the overall v5 UI. File deleted; reference removed. A fresh screenshot is a separate task with its own decisions (mode, sample files, platform chrome).

## Out of scope (deliberately not in this PR)

- New screenshot — separate task.
- Touching the "Development" section. `yarn dev` still launches Electron and that path is functional until Phase G ships; the README's dev instructions match the current dev workflow even if they downplay the Web-first direction.
- Editing `CLAUDE.md`, `project-direction.md`, or `modernization-roadmap.md`. Those are internal docs for contributors and Claude; reframing them is a larger swing.

## Test plan

- [x] `yarn lint` (prettier) passes on README.md
- [ ] Reviewer eyeballs the rendered README in Forgejo's preview to confirm the option ordering and section flow read cleanly
- [ ] No broken intra-repo links (docs/standalone-html.md and the other doc paths all exist)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Randa <obuvuyoviz26@gmail.com>
Reviewed-on: http://localhost:3000/forgejo_admin/exifcleaner-web/pulls/90

README.md

@@ -1,15 +1,13 @@
 # <img src="static/icon.svg" height=26> ExifCleaner
 
-> Clean metadata from images, videos, PDFs, and other files. Desktop app + browser webapp.
-
-![ExifCleaner screenshot](static/screenshot.png)
+> Clean metadata from images, videos, PDFs, and other files. Runs entirely in your browser — no uploads, no server.
 
 ## Features
 
-- Cross-platform: macOS, Windows, and Linux desktop apps + browser-based webapp
+- Browser-based: install as a PWA or run from a single self-contained HTML file — no server, no native install
 - Drag and drop files or folders; recursive folder processing
 - Fully offline — files never leave your device
-- Privacy controls: preserve orientation, save as copy, remove macOS extended attributes, preserve timestamps
+- Privacy controls: preserve orientation, save as copy, timestamps written to epoch
 - 25 translated locales with in-app language switching (contributions welcome in [`.resources/strings.json`](.resources/strings.json))
 - Dark mode (follows OS preference)
 - Free and open source (MIT)
@@ -56,35 +54,21 @@ Footnotes:
 5. MP4/MOV: drops `udta`, `meta`, and `Xtra` containers via mp4box.js box-tree rewrite (no re-encoding, lossless). Known gaps in timed-metadata tracks, `hdlr` names, `compressorname`, mdat orphans, and sidecar files — see [`docs/PRIVACY_GAPS.md`](docs/PRIVACY_GAPS.md#mp4--mov-video-gaps) for the user-facing summary.
 6. RAW: removed in v5 ([issue #16](https://github.com/obuvuyoviz26-lab/exifcleaner-web/issues/16), decided 2026-05-09, shipped 2026-05-10). No production-ready WASM library covers proprietary RAW. RAW workflows should use [ExifTool standalone](https://exiftool.org/) or a dedicated RAW tool — see [`docs/PRIVACY_GAPS.md#raw-unsupported`](docs/PRIVACY_GAPS.md#raw-unsupported).
 
-## Download and Install
-
-macOS 10.15+, Windows 10+, and Linux are supported (64-bit).
-
-- **macOS**: [Download the .dmg file](https://github.com/szTheory/exifcleaner/releases/latest) (universal binary — Intel + Apple Silicon)
-- **Windows**: [Download the .exe installer or portable version](https://github.com/szTheory/exifcleaner/releases/latest)
-- **Linux**: [Download the .AppImage, .deb, or .rpm file](https://github.com/szTheory/exifcleaner/releases/latest)
-
-For Linux, the AppImage needs to be [made executable](https://discourse.appimage.org/t/how-to-make-an-appimage-executable/80) after download.
-
-Arch Linux users can install from the AUR:
-
-```bash
-paru -S exifcleaner-bin
-```
-
-### Verifying checksums
-
-Each release includes a `SHASUMS256.txt` file. Download it from the [release page](https://github.com/szTheory/exifcleaner/releases/latest) and verify your download:
-
-```bash
-sha256sum -c SHASUMS256.txt 2>&1 | grep OK
-```
-
 ## Running the web app locally
 
 ExifCleaner runs entirely in your browser — no server-side processing, no file uploads.
 
-### Option 1: Docker (recommended)
+### Option 1: Single-file HTML (no server, no install)
+
+```bash
+yarn build:web:standalone
+```
+
+Produces `dist/web-standalone/index.html` — one self-contained ~830 KB HTML file with the entire app inlined (JS, CSS, dependencies). Open it directly in any modern desktop browser from disk; no server, no install, no internet. Email it, share it on a USB stick — recipients double-click it and it runs.
+
+This is a desktop deliverable (Chrome, Brave, Edge, Firefox, Safari). Chrome on Android doesn't support opening local HTML files — for mobile, use the PWA install path against a deployed instance. Trade-offs (no service worker, no lazy-loading, no PWA install) and the full rationale live in [`docs/standalone-html.md`](docs/standalone-html.md).
+
+### Option 2: Docker
 
 ```bash
 # Build the image
@@ -96,7 +80,7 @@ docker run -p 8080:80 exifcleaner-web
 
 Open http://localhost:8080. Drag and drop files to clean metadata.
 
-### Option 2: Node dev server
+### Option 3: Node dev server
 
 Requires Node 22 and yarn:
 
@@ -107,7 +91,7 @@ yarn dev:web
 
 Open http://localhost:5173.
 
-### Option 3: Build and preview
+### Option 4: Build and preview
 
 ```bash
 yarn build:web