feat(messages): move find-in-conversation to a searchable field

[garthvh]

What

Replaces the custom magnifying-glass toolbar button for message search with a standard SwiftUI .searchable field, pinned in the navigation bar drawer of channel and direct-message conversations.

Why

.searchable is the platform-standard, discoverable entry point for search — no learning the custom icon, consistent with the rest of iOS, and it frees up the trailing toolbar slot. This builds directly on the find-in-conversation feature added in #2079.

Behavior

The find-in-page navigation is unchanged — only the entry point moved:

• Type in the Find in conversation field below the title to search the conversation.
• A results bar appears while a query is active, showing your position (e.g. 2 of 7) with up/down chevrons to step through matches, wrapping at the ends.
• The current match is highlighted and scrolled into view, lazy-loading older messages as needed.
• Matching stays case- and accent-insensitive over the entire conversation history.
• Cancelling or clearing the field dismisses the results and returns to the conversation.

Notes

• Applies to both ChannelMessageList and UserMessageList.
• MessageSearchBar is now purely the results/navigation bar; the text field is owned by .searchable.
• User docs updated (docs/user/messages.md).

[coderabbitai]

Caution

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