feat(web+api): browser-side per-topic encryption (v0.3.0 phase 3.5)

Closes the v1-vs-v2 split between CLI and dashboard. The web chat
panel now reads and writes the same crypto_secretbox-under-topic-key
ciphertext that CLI 1.8.0+ writes — every encrypted topic finally
renders correctly from the browser.

API
- POST /v1/me/peer-pubkey replaces the throwaway pubkey that
  mutations.ts mints at mesh-create time with one whose secret the
  browser actually holds. Idempotent; auth via the dashboard apikey
  whose issuedByMemberId is the row to update.

Web
- apps/web/src/services/crypto/identity.ts — IndexedDB-backed
  ed25519 identity, lazy-init on first use. Generates once per
  browser-profile; survives reload. ed25519 → x25519 derivation for
  crypto_box decrypt. Module-cached after first call.
- apps/web/src/services/crypto/topic-key.ts — mirrors the CLI
  topic-key service. Fetches GET /v1/topics/:name/key, decrypts the
  sealed copy with our x25519 secret, caches the 32-byte symmetric
  key in-memory keyed by (apikey-prefix, topic). encryptMessage /
  decryptMessage map directly onto crypto_secretbox{,_open}.
- apps/web/src/modules/mesh/topic-chat-panel.tsx — on mount:
  registers our pubkey, fetches the topic key, polls /key every 5s
  while not_sealed (matching the CLI's 30s re-seal cadence). Render
  branches on bodyVersion: v2 -> decrypted-cache, v1 -> legacy
  base64. Send branches: encrypts under the topic key when key is
  ready, falls back to v1 plaintext on legacy or not-yet-sealed
  topics. Composer shows a 🔒 v0.3.0 / "waiting for re-seal" badge.

Adds libsodium-wrappers + @types to apps/web. Browser bundle picks
up its own copy; the existing CLI/broker/API copies are untouched.

Threat model: IndexedDB is per-origin and not exfiltratable from
other sites; XSS or a malicious extension still wins, same as for
any browser-stored secret. Documented divergence from the CLI's
~/.claudemesh-stored keypair in the identity module's preamble.

packages/api/src/modules/mesh/v1-router.ts

@@ -271,6 +271,61 @@ export const v1Router = new Hono<Env>()
     });
   })
 
+  // POST /v1/me/peer-pubkey — register the caller's persistent peer pubkey.
+  //
+  // Browser users get a throwaway ed25519 pubkey at mesh-create time
+  // (no secret retained). To participate in v0.3.0 per-topic encryption
+  // they must replace it with a pubkey whose secret they actually hold
+  // (persisted in IndexedDB). This endpoint writes the new pubkey on the
+  // mesh.member row identified by the apikey's issuer; the broker / CLI
+  // re-seal loop then picks them up as a regular topic-key recipient
+  // within ~30s.
+  //
+  // Idempotent: same pubkey → no-op; different pubkey → updates and
+  // bumps `joined_at` so re-sealers notice the change. We do NOT
+  // invalidate the existing sealed topic_member_key rows here —
+  // they're keyed by member, not pubkey, and the next CLI re-seal pass
+  // will overwrite them with copies sealed to the new pubkey.
+  .post(
+    "/me/peer-pubkey",
+    validate(
+      "json",
+      z.object({
+        pubkey: z
+          .string()
+          .length(64)
+          .regex(/^[0-9a-f]{64}$/i, "must be 64 lowercase hex chars"),
+      }),
+    ),
+    async (c) => {
+      const key = c.var.apiKey;
+      if (!key.issuedByMemberId) {
+        return c.json({ error: "api_key_has_no_issuer" }, 400);
+      }
+      const body = c.req.valid("json");
+      const newPubkey = body.pubkey.toLowerCase();
+      const [existing] = await db
+        .select({ peerPubkey: meshMember.peerPubkey })
+        .from(meshMember)
+        .where(eq(meshMember.id, key.issuedByMemberId));
+      if (!existing) {
+        return c.json({ error: "member_not_found" }, 404);
+      }
+      const changed = existing.peerPubkey !== newPubkey;
+      if (changed) {
+        await db
+          .update(meshMember)
+          .set({ peerPubkey: newPubkey })
+          .where(eq(meshMember.id, key.issuedByMemberId));
+      }
+      return c.json({
+        memberId: key.issuedByMemberId,
+        pubkey: newPubkey,
+        changed,
+      });
+    },
+  )
+
   // GET /v1/topics — list topics in the key's mesh
   // Includes per-topic unread counts when the key has an issuing member
   // (i.e. dashboard keys; CLI-minted keys also carry it). Counts are