feat(tauri-app): profile view, Avatar component, handle navigation

UI half of the profile feature. Mirrors the previous three commits
so the user can browse and edit profiles.

* `<Avatar did cid name size>` — reusable avatar component.
  Falls back to an initial-letter (or "?" when name is empty) circle
  when `cid` is null. Resolves the blob through the standard PDS
  fetch path so it works for any author whose PDS the client can
  reach.
* `<UserProfileView handle on_thread_click current_user_did>` —
  public profile page. Fetches `GET /api/profile/<handle>` on
  mount, renders the avatar / display name / bio / counts /
  posts. The "edit profile" button is gated on
  `current_user_did === profile.did` so a user browsing
  someone else's profile can't issue an unintended `setMyProfile`
  against their own DID.
* `PostCard` now renders an inline `<Avatar>` + clickable handle
  button that calls a new `on_handle_click` prop. The clickable
  area replaces the previous dead `<a href>` (Tauri webviews
  have no router).
* `App.svelte` adds an `openUserProfile(handle)` handler that
  sets `selectedHandle` + `view = "user"` and mounts
  `<UserProfileView>`.
* New Tauri commands `profile_get_record` / `profile_set` in
  `lib.rs` + matching client helpers `getMyProfile` /
  `setMyProfile` in `client.ts`. The set command sends camelCase
  field names; the PDS endpoint (previous commit) round-trips them
  through `#[serde(rename_all = "camelCase")]`.
* Empty-state UX for users with no profile yet (new account, or a
  third-party-PDS author whose profile the AppView hasn't indexed
  yet): both the current-user "profile" view and the public
  "user" view render a hint ("// no profile yet — click 'edit
  profile' to set one up." / "// no profile yet.") instead of a
  blank bio box.
* NavRail / NavRailHarness `View` union extended with "user"
  so the navigation prop type accepts the new view.

crates/tauri-app/src/lib/components/Avatar.svelte

@@ -0,0 +1,92 @@
+<script lang="ts">
+  import { fetchBlob } from "../api/client";
+  import { onDestroy } from "svelte";
+
+  type Props = {
+    did: string;
+    /** Blob-ref `$link` from a posts/post record or a profile
+     *  record. The Avatar component resolves the (did, cid) pair via
+     *  the existing PDS `getBlob` route through `fetchBlob`. NULL
+     *  falls back to the deterministic initial-letter SVG. */
+    cid?: string | null;
+    /** Human-readable name used for the initial-letter fallback and
+     *  the alt text. */
+    name?: string | null;
+    /** Pixel size. The same component is used at 24 px (PostCard),
+     *  32 px (Header current-user avatar) and 88 px (Profile-View
+     *  header). */
+    size?: number;
+  };
+
+  let { did, cid = null, name = "", size = 32 }: Props = $props();
+  const initial = $derived(
+    ((name || "").trim()[0] || "?").toUpperCase(),
+  );
+  let blobUrl: string | null = $state(null);
+  let lastCid: string | null = null;
+
+  $effect(() => {
+    // Drop the previous blob URL when the CID changes — keeps the
+    // in-memory cache (managed by `fetchBlob`) lean and avoids leaking
+    // object URLs across navigations.
+    if (lastCid !== cid) {
+      if (blobUrl) URL.revokeObjectURL(blobUrl);
+      blobUrl = null;
+      lastCid = cid;
+    }
+    if (!cid) return;
+    let cancelled = false;
+    fetchBlob(did, cid)
+      .then((u) => {
+        if (!cancelled) blobUrl = u;
+        else URL.revokeObjectURL(u);
+      })
+      .catch(() => {
+        /* Fall back to the initial letter on fetch error. */
+      });
+    return () => {
+      cancelled = true;
+    };
+  });
+
+  onDestroy(() => {
+    if (blobUrl) URL.revokeObjectURL(blobUrl);
+  });
+</script>
+
+{#if blobUrl}
+  <img
+    class="avatar"
+    style:width="{size}px"
+    style:height="{size}px"
+    src={blobUrl}
+    alt={name ? `${name}'s avatar` : "avatar"}
+  />
+{:else}
+  <span
+    class="avatar avatar--fallback"
+    style:width="{size}px"
+    style:height="{size}px"
+    style:font-size="{Math.max(10, Math.floor(size * 0.45))}px"
+  >
+    {initial}
+  </span>
+{/if}
+
+<style>
+  .avatar {
+    display: inline-block;
+    border-radius: 50%;
+    object-fit: cover;
+    background: var(--bg-elev, #1a1a1a);
+    flex-shrink: 0;
+  }
+  .avatar--fallback {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    font-family: var(--font-mono, monospace);
+    color: var(--text-dim, #888);
+    border: 1px solid var(--line-2, #3a3a3a);
+  }
+</style>