feat(channels): rename channels, and quieter defaults for sound and streams

Four owner requests in one pass.

Rename: the PATCH /api/channels/:id route already accepted a name and a
topic, and the API client already exposed it — the only thing missing was a
way to reach it. The channel settings Overview tab now edits both, gated on
MANAGE_CHANNELS like the server is, with Save/Discard appearing only when
the form is dirty. The server's lowercase-and-hyphenate rule moved to
@backspace/shared so the field can show what a name will actually become
instead of letting it change shape on save; one copy of the rule, used by
both sides.

Stream volume: someone else's screen share now starts at 50 instead of 100
(DEFAULT_STREAM_VOLUME). The publisher boosts system audio before sending,
so the old default arrived far too loud for a viewer who never touched the
slider. Only the default moves — the slider still covers the full range and
a value the user set is still persisted.

Sound effects: SFX_BASE_VOLUME 0.8 -> 0.4, halving every cue. The .ogg files
were left alone; they already peak below full scale, so the harshness was
level, not recorded distortion. Every SFX playback path goes through
getSfxVolume(), so this is the single knob.

Translations: channel settings (including the override editor's permission
labels), the friends page end to end — tabs, lists, requests, the add tab
and discovery cards — the invite modal including its per-invite failure
reasons, and the group-DM member actions. Failure reasons are carried as
translation keys rather than finished text, so results already on screen
re-label when the language changes.

Verified: web build (tsc + vite), server typecheck, 490 web tests, 804
server tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/sfx/README.md

@@ -30,3 +30,12 @@ Para regerar: `python3 <script>.py` produz `.wav`, depois converter com
 
 Ajustes comuns: `HARM` muda o timbre, `tau` a duração, `step` a velocidade do
 arpejo, e o alvo em `write()` o volume (pico RMS).
+
+## Volume de reprodução
+
+O nível com que os arquivos tocam **não** está aqui: está em
+`packages/web/src/utils/sfx.ts`, na constante `SFX_BASE_VOLUME`, multiplicada
+pelo controle de efeitos sonoros do usuário. Em 2026-09-02 ela caiu de `0.8`
+para `0.4` — o dono achou os sons estourados. Os `.ogg` não foram regerados:
+eles já têm pico abaixo do máximo, então o problema era de nível, não de
+distorção gravada. Mexa nessa constante, não nos alvos de RMS daqui.