fix(security): bound + author-key chat attachment cache (Tier C F-02/F-12)

The chat-attachment result cache (`attachment_data` + `image_handle_cache`)
was keyed by attachment id alone and only cleared on room-leave, so an
authenticated insider could (F-02) stream distinct attachments to grow it
without bound, and (F-12) reuse a victim's attachment id to alias displayed/
saved bytes — the id is attacker-chosen, so a signature only proves keypair
ownership, not a distinct human.

F-12: thread the author (`from: EndpointId`) back through the
`AttachmentReady`/`AttachmentFailed` core→UI events (the fetch task already
holds it) and key all attachment result state on `(author, id)`:
- new `AttachmentKey = (EndpointId, AttachmentId)`;
- `attachment_data` + `image_handle_cache` fold into one `AttachmentCache`;
- `pending_saves` and the `SaveAttachment`/`PlayAudio` messages re-keyed, so
  the save/fetch dispatch can't be redirected to the wrong sender's line;
- `find_attachment_source` now matches author AND id;
- the render path resolves each line's key from `ChatEntry.from`.

F-02: `AttachmentCache` is bounded (`ATTACHMENT_CACHE_CAP = 64`) with
insertion-order eviction. True LRU is impossible because iced's `view`
borrows `&self` and so can't reorder on a render read; the generous cap means
a normal session never evicts and the newest (on-screen) entries are always
retained — only an abusive stream hits the bound.

Deliberately id-keyed (cosmetic only, documented): the clip player's
`playing_id`, `pending_plays`, `invalid_audio` — they're coupled to the
id-keyed clip player, and the bytes actually played come from the
author-keyed cache, so content is always correct.

No gossip/wire/protocol change (UiEvent is in-process), no new deps. +6
unit tests (cache eviction, replace-keeps-position, same-id/distinct-author
non-aliasing, is_ready/handle/clear, cap-zero clamp). 423 lib tests,
clippy --all-targets clean, release build green. TESTS-GREEN-ONLY.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/core/messages.rs

@@ -133,11 +133,13 @@ pub enum UiEvent {
     /// string, used to key their avatar (W4).
     ChatMessage { from: String, name: String, text: String, attachment: Option<crate::files::ChatAttachment> },
     /// An attachment's bytes are now available (auto-fetched for images, or
-    /// fetched on demand for files). Keyed by attachment id so the UI can match
-    /// it to the chat entry.
-    AttachmentReady { id: crate::files::AttachmentId, data: Vec<u8> },
+    /// fetched on demand for files). Keyed by `(from, id)`: the id is
+    /// attacker-chosen, so a malicious peer can reuse a victim's id — the author
+    /// disambiguates whose bytes these are and stops content aliasing (Tier C
+    /// F-12).
+    AttachmentReady { from: EndpointId, id: crate::files::AttachmentId, data: Vec<u8> },
     /// An attachment fetch failed (sender gone, too large, decode error, etc.).
-    AttachmentFailed { id: crate::files::AttachmentId, error: String },
+    AttachmentFailed { from: EndpointId, id: crate::files::AttachmentId, error: String },
     /// Our own screen share started; the UI flips the Share button to "Stop".
     ScreenShareStarted,
     /// Our own screen share stopped (or failed to start).