Web: the layer and footage caches stay small enough for eight stacked clips

Cause: the browser build kept the desktop's cache budgets (1 GB layer
cache, 1 GB footage frame cache) inside the page's memory, which wasm32
caps at 4 GiB. FilmCraft's decoders keep up to 384 MB of decoded frames
per open movie beside them, so a comp of eight stacked 1080p H.264 clips
grew the page by about 55 MB/s while playing and ran it out of memory
after about 50 s ("unreachable": a failed allocation).

Fix: in the browser Prefs::cache_budgets keeps the Layer Cache within
512 MB and the Footage Frame Cache within 256 MB (WEB_LAYER_CACHE,
WEB_MEDIA_CACHE); smaller settings and the desktop are unchanged, and
the RAM preview keeps its budget. Docs (web, preferences) say so.

Tests: tests_m145::web_cache_budgets_leave_room_for_the_decoders: with
4 GB settings the web budgets are the caps, the desktop's are the
settings, and smaller settings stay as they are.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

docs/web.md

@@ -61,6 +61,7 @@ device of their own), `?nosw` (don't register the service worker).
 | config directory: settings, shortcut presets, recent projects, the recovery sentinel | `store::WebConfig` in browser storage |
 | auto-save folder | `/EffectCraft Auto-Save/` in browser storage (`ConfigStore::files`) |
 | media reads (`MediaPool`, `probe`) | `files::WebImporter`: bytes from the file table, handed to `MediaPool::add_bytes` / `probe_bytes` |
+| Memory & CPU cache budgets, scaled to the system's RAM | the page's memory, at most 4 GiB (wasm32), which the caches share with the decoders' own decoded frames (FilmCraft keeps up to 384 MB per open movie): the Layer Cache stays within 512 MB and the Footage Frame Cache within 256 MB (`Prefs::cache_budgets`) |
 | export writes (`effectcraft-export`) | the job's `sink` (`effectcraft_host::FileExporter { sink }`): files land in the table and download after the render; several files (an image sequence) download as one stored `.zip` |
 | rfd file dialogs | `<input type=file>` for File ▸ Open / Import; drop files anywhere on the page (`.ecproj` opens, everything else imports); "Save As" / "Output To" pick a download name |
 | Media Browser on the file system | browser storage and folders opened with the File System Access API (below) |