refactor(client): split hooks/builders.rs by event category

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Summary

client/hooks/builders.rs had grown to 574 lines containing ~60
on_* builder methods in one flat file. Split by event category
per the AGENTS.md guideline ("split files when a module grows
beyond ~400-600 lines or mixes multiple responsibilities"):

• builders/mod.rs - module wiring + the catch-all on_event
  hook (keeps its existing spot because it is cross-cutting, not
  tied to a specific event category).
• builders/connection.rs - connect success/fail/crypt/lost,
  max-payload updates, command processing/error/success.
• builders/session.rs - login/logout/kick/user presence
  (logged-in, logged-out, user-update, joined, user-joined,
  user-left, text messages, user-state-change).
• builders/directory.rs - channel/server/file/account/ban
  directory mutations.
• builders/media.rs - audio block/video/desktop/voice-activation/
  hotkey/file-transfer/sound-device events.
• builders/reconnect.rs - internal errors + auto-reconnect/
  auto-login/auto-join recovery pipeline.

Each submodule adds impl ClientHooks methods to the same
ClientHooks struct defined in the parent hooks/ module. No new
public items, no re-exports needed; external call sites
(ClientHooks::new().on_*(...)) continue to compile unchanged.

Structural only, no semantic change:

• All public items keep their paths and visibilities.
• Method bodies are byte-for-byte identical to the pre-split file.
• No new imports beyond what each subcategory needs (e.g.
  session.rs uses ChannelId, TextMessage, User;
  media.rs and reconnect.rs only need Client, Message).

Net diff: builders.rs deleted (574 lines), six new files created
(mod.rs ~45, connection.rs ~75, session.rs ~90,
directory.rs ~105, media.rs ~235, reconnect.rs ~100). Git
detects builders.rs -> builders/mod.rs as the primary rename.

Local verification:

• cargo fmt --all --check clean
• cargo clippy --workspace --all-targets --all-features -- -D warnings clean
• cargo test --workspace --all-features -> 287 passed / 0 failed

Review & Testing Checklist for Human

• Spot-check that ClientHooks::new().on_*(...) chains still
  compile from crates/teamtalk/tests/ and any downstream
  crates. Each on_* method is a #[must_use] builder that
  stores a boxed closure in the matching optional field on
  ClientHooks and returns self; no runtime behavior has
  changed.
• Confirm the category groupings are reasonable:
  connection = transport + command lifecycle;
  session = auth + presence + text;
  directory = channels/server/files/accounts/bans;
  media = audio/video/desktop/sound devices/hotkeys/voice
  activation/file transfer;
  reconnect = internal-error + auto-recovery pipeline.
  Boundaries are subjective; on_event is intentionally kept
  in builders/mod.rs as the one truly cross-cutting hook.
• Verify feature-gated builds (--no-default-features,
  --features async, --features mock, --features all) still
  compile. Local cargo check --all-features is clean; CI
  covers the matrix.

Notes

Next PR in the P0 structural refactor queue after #44
(can_issue_logged_in_command dedup), #45 (poll_command_completion
extract), #46 (client/bus.rs split), #47 (bot/storage.rs split),
and #48 (bot/fsm.rs split). Branches from clean main,
independent of the other refactor PRs, so can be merged in any
order.

Upcoming: split types/entities/media_common.rs (458 lines) and
the client/backend.rs (1334 lines) + client/backend_mock.rs
(1101 lines) pair, then P1 API tweaks (jitter, StreamTypes,
SdkErrorCode, TimeoutKind, SecretString, indexed dispatch)
each with their own tests, then a big test-fill PR driven by
scripts/audit_teamtalk_coverage.py.

The pre-existing semver CI gate is expected to remain red;
release-plz handles the eventual version bump.

Link to Devin session: https://app.devin.ai/sessions/71fdd6196cb74723a2e277bb81993a9c
Requested by: @BlindMaster24

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✅ Devin Review: No Issues Found

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