fix(web): validate inputs in handleConfigSave to prevent fuzz bricking (M1)

Previously /config accepted any value and called ESP.restart() — fuzz tests
(or any malicious POST) could save garbage SSIDs and brick the device until
FTDI recovery. Now:

- SSID: rejected if empty, >31 chars, non-printable, or all-same-char (HTTP 400)
- Password: rejected if >63 chars
- Hostname/city_name/ntp_server: length-validated
- Numeric fields (timezone, brightness, intervals, lat/lon, display): clamped
  to safe ranges via constrain()

Tested on hardware: ssid="AAAA..." now correctly returns HTTP 400 and
preserves existing config. Device survives entire fuzz suite.

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

CHANGELOG.md

@@ -5,6 +5,17 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.9.7] - 2026-05-19
+
+### Fixed
+
+- **Config endpoint accepted garbage values, bricking device** (M1): `/config` form handler
+  now validates all inputs. SSID rejected if empty, >31 chars, non-printable, or all-same-char
+  (fuzz garbage like "AAAA..."). Numeric fields are `constrain()`-ed to safe ranges
+  (`ntp_interval`/`weather_interval`: 60–86400, `brightness`: 0–7, `timezone`: ±12h,
+  `latitude`/`longitude`: physical ranges, `display_orientation`: 0–3). Invalid input
+  returns HTTP 400 instead of silently saving and rebooting.
+
 ## [1.9.6] - 2026-05-18
 
 ### Fixed