fix(mobile): keep #-prefixed markdown link labels visible

[olympusbuildz]

Problem

On iOS, Markdown links whose label starts with # render as nothing. A body like:

See [#2959](https://github.com/block/buzz/issues/2959) for details.

shows See for details. — no label, no underline, no tap target. Desktop renders the same message correctly.

Root cause

1. MessageContent registers _ChannelLinkMd (and mentions/emoji) ahead of gpt_markdown defaults.
2. Default ATagMd builds the link label by re-running the full inlineComponents list on the label text.
3. _ChannelLinkMd matches digits-only tokens (#2959) and replaces them with a _TokenPill WidgetSpan.
4. _buildLink only collects plain TextSpan.text, so the label becomes empty nested placeholders and draws blank on device.

Paths: mobile/lib/features/channels/message_content.dart, message_content/token_pill.dart.

Fix

• _AuthoredMarkdownLinkMd: own the [label](url) path; generate labels with stock formatting components only (no mention/channel/emoji). Drop stock ATagMd from the list to avoid double handling.
• Channel generic token: require at least one letter/underscore so bare digits-only #2959 is never a channel pill (defense in depth for non-link text too).

Why it matters

GitHub-style issue/PR links in chat are common. Invisible links on mobile lose the destination and break reading flow; desktop already works.

Test plan

cd mobile
flutter test test/features/channels/message_content_test.dart
# 76 passed @ fa17ab2ed

Named regressions:

• digits-only bare #token is not a channel pill
• keeps #-prefixed link labels visible (not channel pills)
• known channel name inside a link label stays plain text
• existing #channel links group still green

Manual verify

1. Post See [#2959](https://github.com/block/buzz/issues/2959) for details. from Desktop.
2. Open the message on iOS — label #2959 visible, tappable, opens the URL.
3. Bare #general for a real channel still pills and navigates.

Risk / blast radius

• Link labels no longer turn into channel/mention pills (authored [label](url) keeps plain label text). Bare #channel / @mention outside links unchanged.
• Digits-only bare #N no longer becomes a non-interactive channel pill (never a valid channel name).

Closest work

none found (issue #6124 unlinked; related area #4572 is tappable-links, different symptom)

Fixes #6124