chat: enforce shared text policy at UI, sign point, and gossip ingress

Chat-hardening plan Phase 1. The chat body policy (2,000-char + 8 KiB
ceilings, single-pass control/whitespace normalization) moves from the UI
layer into src/sanitize.rs and is now enforced at every trust boundary:
cap_chat_input bounds the live input (oversized paste), the gossip sign
point re-sanitizes so non-UI callers can't bypass policy, and gossip
ingress rejects oversized raw text before sanitizing (admit_chat_text)
and drops messages with neither visible text nor an attachment. The
incoming chat author label now uses the strict name sanitizer until
Phase 2 roster-binds it. +8 tests (532 lib green).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/widget/selectable_text.rs

@@ -21,7 +21,7 @@ const HIT_SEARCH_STEPS: usize = 24;
 // `Hit::CharOffset(cursor.index)`, and cosmic-text's `cursor.index` is a byte
 // offset WITHIN its buffer line — it discards the line number. That equals the
 // global byte offset only when the text is a single logical line. Chat bodies
-// satisfy this because `app::sanitize_chat` turns every control char (incl. `\n`
+// satisfy this because `sanitize::sanitize_chat` turns every control char (incl. `\n`
 // and `\r`) into a space and collapses whitespace, so a stored message can never
 // contain a newline. If that sanitizer ever starts preserving newlines, this
 // widget's per-line offsets would stop being global and selection/copy across