fix: Gitea push 404 — add branch:main payload, handle 409 empty repo, clearer settings

- lib/git.ts: add branch:"main" to Gitea POST payload (required when repo has no
  branch yet or is freshly initialized); treat 409 on GET same as 404 (empty repo
  has no tree, Gitea returns 409 Conflict — proceed to create first file)
- lib/git.ts: improve 404 error hint with actionable checklist (URL format,
  owner/repo names, token scope, repo existence)
- settings.tsx: add Gitea info card clarifying URL format (no /api/v1), token
  format (raw value only, no "Bearer" prefix), repo initialization requirement

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/settings.tsx

@@ -402,6 +402,25 @@ export default function SettingsScreen() {
 
       {gitProvider === "gitea" && (
         <>
+          <View
+            style={{
+              backgroundColor: Colors.surface,
+              borderRadius: 8,
+              padding: 12,
+              marginBottom: 14,
+              borderLeftWidth: 3,
+              borderLeftColor: Colors.text2,
+            }}
+          >
+            <Text style={{ color: Colors.text2, fontSize: 12, lineHeight: 18 }}>
+              <Text style={{ fontWeight: "700" }}>Instance URL</Text>
+              {" — base URL only, no trailing slash, no /api/v1.\n"}
+              <Text style={{ fontWeight: "700" }}>Token</Text>
+              {" — Gitea PAT with write:repository scope. Paste the token value only (no \"Bearer\" prefix).\n"}
+              <Text style={{ fontWeight: "700" }}>Repository</Text>
+              {" — must have at least one commit (initialize it with a README if empty)."}
+            </Text>
+          </View>
           <Field
             label="Instance URL"
             value={giteaUrl}
@@ -410,10 +429,10 @@ export default function SettingsScreen() {
             url
           />
           <Field
-            label="Token"
+            label="Token (PAT, write:repository)"
             value={giteaToken}
             onChange={setGiteaToken}
-            placeholder="Bearer token"
+            placeholder="paste token value here"
             secure
           />
           <Field