Autocomplete channel names when typing # in composer

[loren-magnuson]

Problem
When a user types # in the composer, channel mentions are currently manual. In rooms with many channels, that makes discovery and accurate referencing slower than it needs to be.

Requested behavior

• when the user types #, show channel-name autocomplete suggestions
• filter suggestions as more characters are typed
• insert the selected channel mention into the composer
• keep this scoped to channel names; do not bundle broader mention/autocomplete work unless needed

Acceptance criteria

• typing # in the composer opens a suggestion list of known channels
• typing additional characters narrows the list case-insensitively
• selecting a suggestion inserts the channel reference at the cursor position
• keyboard navigation and Enter/Tab selection work
• escaping or clicking away dismisses the suggestion list
• existing plain-text composer behavior remains intact when no suggestion is chosen

Implementation notes

• likely UI surface is in the composer/input handling path under static/
• reuse any existing mention/autocomplete patterns if present; otherwise keep the first slice narrow and local to channel references