refactor: Simplify new setting introduction (#1086)

## Summary

* **What is the goal of this PR?** Eliminate the 3-file / 4-location
overhead for adding a new setting. Previously, every new setting
required manually editing JsonSettingsIO.cpp in two places (save +
load), duplicating knowledge already present in SettingsList.h. After
this PR, JsonSettingsIO.cpp never needs to be touched again for standard
settings.
* **What changes are included?**
* `SettingInfo` (in `SettingsActivity.h`) gains one new field: `bool
obfuscated` (base64 save/load for passwords), with a fluent builder
method `.withObfuscated()`. The previously proposed
`defaultValue`/`withDefault()` approach was dropped in favour of reading
the struct field's own initializer value as the fallback (see below).
* `SettingsList.h` entries are annotated with `.withObfuscated()` on the
OPDS password entry. The list is now returned as a `static const`
singleton (`const std::vector<SettingInfo>&`), so it is constructed
exactly once. A missing `key`/`category` on the
`statusBarProgressBarThickness` entry was also fixed — it was previously
skipped by the generic save loop, so changes were silently lost on
restart.
* `JsonSettingsIO::saveSettings` and `loadSettings` replace their ~90
lines of manual per-field code with a single generic loop over
`getSettingsList()`. The loop uses `info.key`,
`info.valuePtr`/`info.stringOffset`+`info.stringMaxLen` (for char-array
string fields), `info.enumValues.size()` (for enum clamping), and
`info.obfuscated`.
* **Default values**: instead of a duplicated `defaultValue` field in
`SettingInfo`, `loadSettings` reads `s.*(info.valuePtr)` *before*
overwriting it. Because `CrossPointSettings` is default-constructed
before `loadSettings` is called, this captures each field's struct
initializer value as the JSON-absent fallback. The single source of
truth for defaults is `CrossPointSettings.h`.
* One post-loop special case remains explicitly: the four `frontButton*`
remap fields (managed by the RemapFrontButtons sub-activity, not in
SettingsList) and `validateFrontButtonMapping()`.
* One pre-loop migration guard handles legacy settings files that
predate the status bar refactor: if `statusBarChapterPageCount` is
absent from the JSON, `applyLegacyStatusBarSettings()` is called first
so the generic loop picks up the migrated values as defaults and applies
its normal clamping.
* OPDS password backward-compat migration (plain `opdsPassword` →
obfuscated `opdsPassword_obf`) is preserved inside the generic
obfuscated-string path.

## Additional Context
Say we want to add a new `bookmarkStyle` enum setting with options
`DOT`, `LINE`, `NONE` and a default of `DOT`:

1. `src/CrossPointSettings.h` — add enum and member:
```cpp
enum BOOKMARK_STYLE { BOOKMARK_DOT = 0, BOOKMARK_LINE = 1, BOOKMARK_NONE = 2 };
uint8_t bookmarkStyle = BOOKMARK_DOT;
```

2. `lib/I18n/translations/english.yaml` — add display strings:
```yaml
STR_BOOKMARK_STYLE: "Bookmark Style"
STR_BOOKMARK_DOT: "Dot"
STR_BOOKMARK_LINE: "Line"
```
(Other language files will fall back to English if not translated. Run
`gen_i18n.py` to regenerate `I18nKeys.h`.)

3. `src/SettingsList.h` — add one entry in the appropriate category:
```cpp
SettingInfo::Enum(StrId::STR_BOOKMARK_STYLE, &CrossPointSettings::bookmarkStyle,
{StrId::STR_BOOKMARK_DOT, StrId::STR_BOOKMARK_LINE, StrId::STR_NONE_OPT},
"bookmarkStyle", StrId::STR_CAT_READER),
```

That's it — no default annotation needed anywhere, because
`bookmarkStyle = BOOKMARK_DOT` in the struct already provides the
fallback. The setting will automatically persist to JSON on save, load
with clamping on boot, appear in the device settings UI under the Reader
category, and be exposed via the web API — all with no further changes.

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