Fix config read-modify-write race and two firmware buffer edge cases

Found by a thorough code review:

- server/app/config.py's load()/save() each locked only their own file
  I/O, not the full read-modify-write cycle each route does around
  them. Since uvicorn dispatches sync routes to a thread pool, two
  concurrent requests (e.g. the device's own poll landing alongside a
  web UI edit) could each load() the same on-disk state and the
  second's save() silently clobber the first's changes. Added
  config.locked() (backed by an RLock, since load()/save() also take
  the lock internally) and wrapped every mutating route's
  load/mutate/save span in it -- kept outside the lock wherever a route
  also does slow Immich network I/O, re-loading fresh state right
  before the actual mutation instead. Verified with a new concurrency
  stress test (many concurrent /api/queue/promote and /api/config
  calls) alongside the existing scratch suite.
- firmware/main/root.html's SSID/password/toolsserver/access-token
  inputs had no maxlength, so pasting something longer than the
  matching NVS buffer (wifi_provisioning.h's FRAME_CFG_*_MAX_LEN) was
  silently truncated with no indication why the device later can't
  connect or gets 401s.
- frame_client.c's share_url buffer (256 bytes) could be too small in
  the worst case -- toolsserver (128) + "/frame/share/" + asset_id (47)
  + "?token=" + access_token (64) can reach ~266 bytes, silently
  dropping the token off a request that would then just 401 with no
  obvious cause. Widened to 320.

firmware/main/root.html

@@ -76,26 +76,30 @@
         <p>Please connect me to your local wifi network</p>
         
         <!-- The action attribute should point to wherever your server handles the data -->
+        <!-- maxlength on each field below mirrors its NVS buffer size in
+             wifi_provisioning.h (FRAME_CFG_*_MAX_LEN) -- firmware silently
+             truncates past that length, so keep these in sync if those
+             change. -->
         <form action="/save_config" method="POST">
             
             <div class="input-group">
                 <label for="ssid">Wifi SSID</label>
-                <input type="text" id="ssid" name="ssid" placeholder="Network Name" required>
+                <input type="text" id="ssid" name="ssid" placeholder="Network Name" maxlength="32" required>
             </div>
 
             <div class="input-group">
                 <label for="password">Password</label>
-                <input type="password" id="password" name="password" placeholder="Network Password">
+                <input type="password" id="password" name="password" placeholder="Network Password" maxlength="64">
             </div>
 
             <div class="input-group">
                 <label for="toolsserver">Tools Server</label>
-                <input type="text" id="toolsserver" name="toolsserver" placeholder="e.g. 192.168.1.50:8080 or https://frame.example.com" required>
+                <input type="text" id="toolsserver" name="toolsserver" placeholder="e.g. 192.168.1.50:8080 or https://frame.example.com" maxlength="128" required>
             </div>
 
             <div class="input-group">
                 <label for="access_token">Access Token (optional)</label>
-                <input type="text" id="access_token" name="access_token" placeholder="only if the server's MANAGEMENT_TOKEN is set">
+                <input type="text" id="access_token" name="access_token" placeholder="only if the server's MANAGEMENT_TOKEN is set" maxlength="64">
             </div>
 
             <button type="submit">Submit</button>