Show name and avatar for bot messages

[Frodotus]

Closes #76.

Problem

Bot/app messages (bot_message) carry no user — only bot_id, username, and sometimes icons. The conversion read only m.User, so bot messages got UserID == "": the avatar lookup missed and the author name was dropped. This hit every bot, since even apps with a bot user post as bot_message with an empty user.

Fix

• Identity: key bot messages on bot_id and use the message's username, in all three fetch paths (new messageAuthor helper) and the live WebSocket path (OnMessage now receives bot_id/username, threaded through the event struct + dispatch).
• Avatar: a new userResolver.RequestBot resolves the icon via bots.info once per bot (deduped, persisted) and preloads it via avatars.Preload(bot_id, …); AvatarURLs[bot_id] is then populated on the resolve/connect path — mirroring the users.info path for humans. Added GetBotInfo to the client.
• Cached rows: messages cached with UserID == "" before this change are re-keyed by parsing bot_id/username out of raw_json in enrichCachedRow, so existing cached bot messages render without a cache wipe.

Testing

go test ./... passes. Added TestMessageAuthorBotIdentity (fetch-path identity) and extended TestDispatchWebSocketBotMessageEvent to assert bot_id/username flow through the WS dispatch.

[gammons]

Reviewed in a worktree — build passes, go vet clean, all tests green (including the two new bot-identity tests). Approve with minor nits.

Well-targeted fix: correct root cause (bot_message has empty user, only bot_id/username), consistently applied across all three fetch paths, the live WS path, and the cached-row backfill. RequestBot faithfully mirrors Request and the avatar pipeline wires through unchanged since cache + MessageItem are keyed on the same bot_id. The enrichCachedRow re-keying for pre-fix rows (avoiding a cache wipe) is a nice touch. Tests are meaningful, not signature-padding.

Nits (non-blocking):

• bestBotIcon comment says "largest reasonable icon," but the fallback order (72, 48, 132, 36, 230) deliberately prefers 72 over the larger sizes. The ordering is fine; the comment should reflect the actual intent.
• ShouldNotify uses the original userID (empty for bots) rather than authorID — almost certainly intentional (a bot cant be you), but worth a one-line comment so it is not "fixed" later.
• PR description says the avatar is stored into AvatarURLs[bot_id]; precisely, RequestBot calls avatars.Preload(botID, ...) and AvatarURLs is populated on the UserResolvedMsg/connect path. Functionally correct, just imprecise wording.

[Frodotus]

Thanks for the review! Addressed the nits in f387eb9 (non-functional, comments + wording only):

• bestBotIcon comment — rewritten to match the actual intent: prefer the 72px icon (sharp enough, almost always present) and fall back through the other sizes by availability, not strictly by pixel size.
• ShouldNotify userID — added a one-line comment at the call site noting the empty userID for bots is intentional: the self-message suppression keys on the human sender, and a bot (userID == "", authorID == botID) can never be "you".
• PR description — corrected the avatar wording: RequestBot calls avatars.Preload(bot_id, …), and AvatarURLs[bot_id] is populated on the resolve/connect path (not stored directly by RequestBot).