feat: PWA support for the web client (#5184)

* feat: PWA support for the web client

Adds Progressive Web App support so the Zellij web client can be
installed as a standalone app via Chromium/Firefox/Safari/iOS.

- New static asset `zellij-client/assets/manifest.webmanifest` with
  required W3C members (name, start_url, scope, display, icons). All
  path-bearing fields are relative to the manifest URL so a single
  static file works under any reverse-proxy base_url prefix without
  runtime templating.
- New icon set under `zellij-client/assets/`: icon.svg (vector),
  icon-192.png and icon-512.png (purpose "any"), and
  icon-maskable-512.png (purpose "maskable") rendered at 70% of
  canvas height on a #080317 backdrop so Android adaptive masks
  honour the W3C 80%-diameter safe zone.
- `scripts/render-pwa-icons.sh` pins the rendering toolchain
  (librsvg + ImageMagick) and the maskable safe-zone geometry so
  re-renders are reproducible.
- `.webmanifest` -> `application/manifest+json` mapping added to
  `get_mime_type` in `zellij-client/src/web_client/utils.rs`.
- `index.html` gains <link rel="manifest">, apple-touch-icon, and
  apple-mobile-web-app-* meta tags. Placed immediately after the
  existing favicon link and before any <script> tag so the manifest
  is in the browser's preload scanner early.
- 5 new tests cover Content-Type, required JSON members, the
  relative-URL contract, index.html references, icon reachability,
  and base_url invariance.

Closes #5094.

* update changelog with pull url

* feat(web): trim PWA icon set to PNGs only

Drops the duplicate `zellij-client/assets/icon.svg` (verbatim copy of
`assets/logo.svg`) and the maskable PNG variant from the manifest.
Net result is the minimal icon set most production PWAs ship:
icon-192.png and icon-512.png.

- `icon.svg` was redundant with the canonical `assets/logo.svg` at
  the repo root and offered no install-criterion benefit on top of
  the 192/512 PNGs.
- `icon-maskable-512.png` only benefits Android Chrome installs
  (desktop browsers and iOS ignore the maskable purpose). The
  Android win was small for the hexagonal Zellij glyph; a hand-tuned
  maskable can be added in a follow-up if Android installs need
  polish.

`scripts/render-pwa-icons.sh` now reads from `assets/logo.svg`
directly instead of the duplicate copy.

* feat(web): use existing assets/logo.png as the PWA icon

Removes the pre-rasterized 192/512 PNG variants and the render
script in favour of serving the canonical `assets/logo.png` via a
small dedicated handler. The repo's existing logo becomes the single
source of truth; the PWA picks up logo changes automatically with no
re-render step.

- `LOGO_PNG` static is loaded via `include_bytes!` from the repo
  root, mirroring how `WEB_CLIENT_PAGE` reaches `assets/index.html`.
- New `serve_logo_png` handler is registered at `/assets/logo.png`
  before the `get_static_asset` wildcard. axum's matchit router
  gives literal paths precedence, so the explicit route always wins.
- Manifest declares a single icon entry at the actual file dimensions
  (2269x2620). Browsers letterbox onto square launcher surfaces, the
  same way they do for the existing favicon.
- `apple-touch-icon` retargets to `assets/logo.png`.
- `test_pwa_icon_assets_reachable` collapses to a single case.
- `scripts/render-pwa-icons.sh` and the rendering toolchain
  (librsvg + ImageMagick) are no longer needed.

* feat(web): drop redundant apple-mobile-web-app meta tags

The three `apple-mobile-web-app-*` meta tags are backward-compat
scaffolding for iOS < 16.4:

- `apple-mobile-web-app-capable: yes` — iOS 16.4+ reads
  `display: standalone` from the manifest.
- `apple-mobile-web-app-title: Zellij` — iOS 16.4+ reads `short_name`
  from the manifest.
- `apple-mobile-web-app-status-bar-style: default` — explicitly sets
  iOS's default status-bar behaviour, i.e. a no-op.

iOS 16.4 shipped in March 2023; the cohort still on iOS 15 is
essentially devices stuck on the iPhone 6s / 7 / SE 1st gen line
(2015-16 hardware), which is not the zellij audience.

`<link rel="apple-touch-icon">` is kept because iOS still ignores
manifest `icons` entries and reads only that tag for the home-screen
icon on every iOS version.

* feat(web): restore square pre-rasterized PNG icon for installability

The earlier swap to serving `assets/logo.png` directly fails
Chromium's PWA installability gate: the logo is 2269x2620 (not
square), and Chromium specifically requires "an icon at least 144
pixels square" -- a non-square icon is rejected even though the
declared `sizes` covers the dimension threshold. DevTools'
Application -> Manifest panel was misleading: it parsed and
displayed the icon, but installability is a separate stricter check.

This walks the swap back to a single committed 192x192 square PNG
(`icon-192.png`) rendered from `assets/logo.svg` with the logo
composited onto a transparent square canvas. The PNG is served by
the existing `get_static_asset` route via `include_dir!`. The
`LOGO_PNG` static, `serve_logo_png` handler, and dedicated
`/assets/logo.png` route are dropped.

apple-touch-icon retargets to icon-192.png (closer to iOS's
preferred 180px home-screen size).

Net effect on the diff: ~13KB committed PNG + a single icon entry
in the manifest, in exchange for actually meeting the
installability criterion.

* style: rustfmt

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Co-authored-by: Robert Jin <robert.jin@gresearch.co.uk>
Co-authored-by: Aram Drevekenin <aram@poor.dev>