feat: add dynamic channel management (/join, /part)

[KinseyD]

Summary

Channels are no longer config-only. This PR adds runtime channel management — /join and /part — with per-channel server confirmation, while the channels config key becomes the startup join intent.

Commands

Command	Behavior
/join #channel	Join or focus a channel on the current registered server; a new JOIN waits for server confirmation.
/part [#channel] [reason]	Leave the named channel, or the current channel when omitted; the buffer and its history are retained.

Both are online-only: the server must have completed registration. Channel names follow the configuration rule — a single # or & prefix with at least one character; whitespace, control characters, commas and colons are rejected. No channel keys or multi-target entries.

Behavior

• Each JOIN and PART has its own 30-second confirmation timeout; the status dot reflects the result (blinking grey while unconfirmed, red when not joined or membership is uncertain after a timeout).
• Runtime membership is memory-only: the config file is never rewritten and runtime changes disappear on exit.
• Reconnection rejoins the current join intent once (including runtime /joins, excluding parted channels).
• KICK keeps join intent without auto-retrying; an unsolicited own PART clears it.
• Channel chat is gated on the confirmed JOIN; a prioritized PART prevents queued channel chat from leaking to the wire.
• Raw console JOIN/PART lines go through the same managed state — they cannot bypass runtime intent and produce no console echoes.
• Opening a configured channel's buffer does not join it by itself.
• PART never deletes history: the buffer, draft and reading position survive.

Implementation

• src/connection/channels.rs (new) — per-channel confirmation state machine in the worker; deadlines and errors scoped to the operation.
• src/application/channels.rs + channel_tests.rs (new) — session join/part intent, buffer lifecycle and status queries.
• src/command.rs — /join / /part parsing (reason preserved, invalid names rejected with the draft retained).
• src/protocol.rs — structured channel errors (code, target, reason).
• src/tui/render.rs — extended status-dot semantics.
• src/config.rs — validate configured channel names at load time.
• Drops the channel-lists feature from the irc dependency; channel state is now managed locally. No new dependencies.

Tests

• New tests/dynamic_channels.rs: 16 integration tests over a mock IRC server on 127.0.0.1 — join waits for own confirmation, independent JOIN/PART deadlines, late confirmations, kick vs. unsolicited PART, reconnect intent, raw-command bypass prevention, offline rejection, and buffer/draft survival.
• Existing suites updated where semantics changed.
• Full suite: 329 passed / 0 failed on Windows; cargo fmt --check and cargo clippy --all-targets -- -D warnings clean.
• Includes the follow-up fix restoring blocking mode on accepted mock sockets — on Windows an accepted socket inherits the listener's non-blocking mode, which made read_line return WouldBlock instead of honoring the read timeout.

Docs

• README: new "Dynamic channels" section, command-table entries, config semantics for channels, and the extended status-dot legend.
• config.example.toml: documents channels as startup join intent.