feat(composer): support link previews on message edit (Option B, follow-up to #5245)

[tellaho]

Context

Follow-up to PR #5245 (link-preview snapshot/send race fix). That PR chose Option A: edit mode is excluded entirely from link-preview resolution, upload, and Save-gating — editing a message shows no card, no spinner, and cannot add or change a link preview.

Option B is the deferred alternative: let message edits add, replace, or remove link-preview snapshots, carrying snapshot tags through the edit path end-to-end.

Scope

• Carry snapshot tags through submitMessageEdit.ts so an edit can persist new preview tags.
• Edit-transport tag-split in hooks.ts (whatever splits edit tags from content on the wire).
• Timeline overlay merge in formatTimelineMessages.ts so an edited message's preview tags override the original's.
• Replacement/removal overlay rules: editing a link out should drop its preview; changing the link should swap the snapshot.
• Re-enable useComposerLinkPreviews in edit mode (Option A currently disables it via editTarget == null) with the same debounce, per-media degrade, and submit-guard behavior as normal send.
• Edit-mode e2e coverage mirroring the normal-send tests: resolve-on-edit, upload-failure degrade, remove-link-drops-preview.

Notes

• Stacks AFTER Option A (PR fix(link-preview): reliably render previews sent right after they resolve #5245) merges.
• Product decision and the full defect/decision history live in the room canvas for channel buzz-link-ograph-embeds.