feat(chat): formatting tools in the chat composer

[bmc08gt]

Adds the composer's formatting tools, behind a new ChatFormattingComposer feature flag (off by default, visible in the staff menu). Rendering stays unflagged.

Stacked on #1748: the dimmed markers and the link sheet's validation call parseChatText, which that PR adds. Retarget to code/cash once it merges.

• Single line: B, I, S, </> and Link lead the system text toolbar as short text labels, through a TextContextMenuProvider over a floating ActionMode (Figma node 11278:806). Items are disabled when the parser would not take the markers (a selection that starts mid-word).
• Two or more lines: an Aa button joins the + in the controls row and hides when the field is one row again. It opens a strip docked under the composer with B, I, S, code, link, bulleted, numbered, quote and code block (node 11272:844). SelectableIconButton and ComposerFormatStrip are new ui/components pieces.
• Wrapping and toggling live in :libs:text-format (toggleInline, toggleLine, insertLink, consumedRanges), pure and unit-tested. Whitespace at the selection's ends stays outside the markers, a second tap removes them, no selection inserts the pair with the cursor between (which arms the style), and line buttons add or remove a prefix on every touched line.
• Link opens a small sheet and accepts only exactly one detected link that the parser would also mask (maskableUrlOrNull).
• Markers stay visible in the field, dimmed, through an OutputTransformation on ChatInput.

Tests: ComposerFormattingTest (18 cases, JVM and iOS-simulator targets) and ComposerLinkTest in :apps:flipcash:shared:chat-ui.