Convert file data strings to actual files

[devmount]

Description of the Change

Converts the logo and signature settings from a plain text cell holding a pasted data:image/...;base64,... string into proper Filament image uploads.

• logo/signature settings are now marked with a new type = 'image'
• In the settings table, those two rows now show an image thumbnail (or a "click to upload" hint when empty)
• Uploaded files are stored on the local disk under settings/; Setting::value now holds the relative file path instead of a base64 string. Setting deletes the previous file automatically whenever an image setting's value is replaced or cleared.
• The logo and signature are now scaled by height only, with width following the uploaded image's own aspect ratio, instead of a hardcoded width/height box.

Benefits

• No more manually pasting a huge base64 string into a single-line text field.
• Upload, replace, and delete are now explicit, discoverable actions with a live preview, instead of an implicit side effect of editing raw text.
• Non-square logos are no longer squashed into a fixed box.

Notes for reviewers / deploy

• Existing base64 values are not auto-converted. After this deploys, logo/signature will appear empty until re-uploaded once through the new UI — this was a deliberate choice to avoid a one-way, unreviewable data migration. PdfTemplate handles the gap gracefully (PDFs generate without a logo/signature in the meantime).
• Because logo width is no longer capped, a very wide/landscape logo could now visually run into the title text in the PDF header. Best to upload a roughly square logo.

Applicable Issues

Closes #193

[devmount]

lgtm