feat(desktop): Feed + Wallet sections (v2.2.0-alpha6)

Desktop client reaches full feature parity with mobile for the two
heaviest sections. Contacts + Devices screen + polish pass remain for
rc1.

Feed section (src/sections/feed/ + src/lib/feed.ts):
  * Left pane — FeedTabs: For You / Following / Trending 24h + a
    hashtag input that promotes to a tab on Enter; breadcrumb back-
    navigation when you drill into an author wall or hashtag.
  * Right pane — FeedPane: two sub-columns. Scrollable post list
    (truncated body, likes/views/hashtags footer, active highlight)
    + PostDetail with full body, hashtag links (click → hashtag tab),
    inline attachment image, like/unlike button, Delete (if mine).
    On-mount side-effects: bumpView + fetchStats for liked-by-me.
  * ComposeModal — new-post dialog. Ctrl/Cmd+N opens it; Ctrl+Enter
    submits. Byte counter against 4000 limit, live hashtag preview.
    Uses publishAndCommit (server-side image scrub happens when
    attachments land in rc1).
  * lib/feed.ts — full mirror of mobile's feed.ts:
    fetchForYou/Timeline/Trending/Author/Hashtag/Post/Stats,
    bumpView, like/unlike/delete/follow/unfollow, publishPost +
    publishAndCommit + buildCreatePostTx. Uses window.crypto.subtle
    for SHA-256 (no expo-crypto dep). Same canonical-bytes as mobile.

Wallet section (src/sections/wallet/ + new bits in src/lib/api.ts):
  * WalletOverview (left): account card (balance + shortened pub +
    Send/Receive/Refresh) and transaction history grouped by row.
    Amount colour-codes by direction; pretty tx-type labels.
  * WalletDetailPane (right): selected tx — big signed amount,
    2-column key/value grid (id, from, to, amount, fee, time, block,
    gas), collapsible JSON payload + payload_hex fallback. Mirror of
    mobile /tx/[id] layout.
  * SendModal — transfer tx with @username / DC-address / hex pub
    resolution via resolveAccount. Balance + fee preview; refuses
    self-transfer (would roundtrip through mempool for no reason).
  * ReceiveModal — pub + Copy button. QR in rc1 once we pull in a
    qrcode lib.
  * lib/api.ts: TxRow + TxDetail types, getTxHistory, getTxDetail,
    resolveAccount (handles hex/@username/DC-address).

Store adds feedTab + feedSelectedPost + walletSel so selection state
survives section-switches. FeedTab discriminated union covers the
hashtag + author sub-states so breadcrumbs know what to render.

Typecheck + renderer build both pass. Node API used as-is — no
server changes in this release.

desktop/package.json

@@ -1,6 +1,6 @@
 {
   "name": "dchain-desktop",
-  "version": "2.2.0-alpha5",
+  "version": "2.2.0-alpha6",
   "description": "DChain desktop client — Electron shell mirroring the mobile app's functionality with a keyboard-first 3-panel layout.",
   "private": true,
   "main": "dist-electron/main.js",