feat: add jump-to-message with highlight animation

[sanasol]

Summary

Adds the ability to click message links (e.g. # General › 💬 badges in markdown) and jump to the referenced message with a visual highlight.

Routing:

• SwitchChannelToMessage action in ActionChannel for message-specific navigation
• ChatRouterScreen routes the action, sets pendingScrollToMessageId state
• ChannelScreen receives pending scroll ID and triggers scroll + highlight

Link parsing (JBMRenderer):

• Parse internal channel/message URLs (/server/ID/channel/ID/ID)
• Resolve pretty display text for links (e.g. show # General instead of full URL)
• Background color badge styling for internal links

Scroll + highlight:

• scrollToItem(index + 1) then animateScrollBy(-viewportHeight / 2f) to center the target in reverseLayout
• If message not in current view, jumpToMessage() loads messages around the target from API
• Pulse highlight overlay via drawWithContent (3 pulses at 0.55→0.20 alpha, settle at 0.30, 3s hold, 1s fade out)

ViewModel enhancements:

• isInMiddleOfHistory / reachedLatest / newestLoadedMessageId state for bidirectional pagination
• jumpToMessage(), returnToLatest(), loadNewerMessages() methods
• switchChannel() now accepts optional targetMessageId for initial scroll
• around parameter support in loadMessages() with proper sort order

Fixes #23, related to stoatchat/for-web#891

Tested and live on https://chat.sanhost.net — app at https://chat.sanhost.net/downloads/

[sanasol]

Note: The second commit also fixes a pre-existing bug (not introduced by this PR) where the channel would sometimes appear empty on fresh app start.

Root cause: RealtimeSocket.connect() calls pushReconnectEvent() on every WS connection, including the first one. This triggers loadMessages(50, ignoreExisting=true) in the Reconnected handler. When the initial loadMessages(50) has already loaded the same messages, ignoreExisting filters them all out → updateItems([]) wipes the channel.

Fix: Early return when ignoreExisting yields no new messages, preventing the empty updateItems() call.