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>

packages/server/src/routes/channels.ts

@@ -15,6 +15,11 @@ import type {
   Channel,
   ChannelCategory,
 } from '@backspace/shared';
+import {
+  normalizeChannelName,
+  CHANNEL_NAME_MIN_LENGTH,
+  CHANNEL_NAME_MAX_LENGTH,
+} from '@backspace/shared/src/constants.js';
 
 function rowToChannel(row: typeof schema.channels.$inferSelect): Channel {
   return {
@@ -190,8 +195,8 @@ export async function channelRoutes(app: FastifyInstance): Promise<void> {
       return reply.code(400).send({ error: 'Channel name is required', statusCode: 400 });
     }
 
-    const trimmedName = name.trim().toLowerCase().replace(/\s+/g, '-');
-    if (trimmedName.length < 1 || trimmedName.length > 100) {
+    const trimmedName = normalizeChannelName(name);
+    if (trimmedName.length < CHANNEL_NAME_MIN_LENGTH || trimmedName.length > CHANNEL_NAME_MAX_LENGTH) {
       return reply.code(400).send({ error: 'Channel name must be between 1 and 100 characters', statusCode: 400 });
     }
 
@@ -296,8 +301,8 @@ export async function channelRoutes(app: FastifyInstance): Promise<void> {
     const updates: Partial<typeof schema.channels.$inferInsert> = {};
 
     if (name !== undefined) {
-      const trimmedName = name.trim().toLowerCase().replace(/\s+/g, '-');
-      if (trimmedName.length < 1 || trimmedName.length > 100) {
+      const trimmedName = normalizeChannelName(name);
+      if (trimmedName.length < CHANNEL_NAME_MIN_LENGTH || trimmedName.length > CHANNEL_NAME_MAX_LENGTH) {
         return reply.code(400).send({ error: 'Channel name must be between 1 and 100 characters', statusCode: 400 });
       }
       updates.name = trimmedName;