fix: bug sweep — profile, mentions, deep links, channel links, doctor page

[wesbillman]

Summary

• Hide "Message" button on own profile (mobile) — added currentPubkeyProvider check to conditionally hide the button when viewing your own profile sheet
• Fix multi-word @mention highlighting (mobile) — pre-process content to replace spaces with NBSP inside known mention names so gpt_markdown's combined regex matches multi-word names as single tokens; normalize back on lookup
• Fix channel link not highlighted at message start (mobile) — prepend ZWS when content starts with #[word-char] to prevent gpt_markdown block processing from swallowing the channel link pattern
• Force workspace reinit on deep link to same relay (desktop) — bump reinitKey in switchWorkspace when id === activeId so deep links properly reconnect even when the workspace is already active
• Clean up Doctor settings page (desktop) — remove binary paths section and "Setup help" section that aren't useful in production builds

Test plan

• dart format clean, dart analyze clean
• tsc --noEmit clean, biome check clean
• All pre-commit hooks pass (desktop, mobile, web, rust)
• 201 unit tests pass (148 sprout-core + 53 sprout-auth)
• Manual: verify own-profile sheet hides Message button
• Manual: verify multi-word @mention highlights fully in message list
• Manual: verify #channel-name highlights when it's the only message content
• Manual: verify deep link to already-active relay reconnects properly
• Manual: verify Doctor page shows only relevant info

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