Add the Neutral template family, template picker and text-only logo allowances

Linea (the vendor's original format.pdf design), Monogram and Serenity are
rebuilt from the reference PDFs as one parametric family on the engine, with
the user's logo in the measured slot, a home for every GST field and
A4/Letter support. Adds static Montserrat, Open Sans and Poppins fonts and
a generated Ms Madi outline for Serenity's vertical word. Text-only logo
placeholders (Monolith, Tangerine) use a doc-derived allowance box. A
Carbon TemplatePicker with committed thumbnails opens from the page
setup controls.

docs/templates/04-linea.md

@@ -139,3 +139,20 @@ Page is 595.5 x 842.25pt (the reference design's A4). In react-pdf use A4 (595.2
 - **Byte-identical page content to /media/Toto/Documents/bongbetic/invoice/format.pdf.** It is the same reference page; only the Info/XMP title, export timestamps, reference design ID, xref offsets and trailer /ID differ. So this IS the vendor's original format, still carrying the reference design's sample data (Daniel Gallego / Richard Sanchez / Borcele Bank).
 - **Not a duplicate of the in-app template.** The current src/pdf/InvoiceDocument.tsx only loosely adapts it (see group_notes).
 - **Sibling of 5.pdf and 6.pdf.** They share the grid: rules 69.64-525.86, column centres 325.2 / 416.4, money right edge 514.4, meta label/value anchors about 447 / 519, totals label anchor 460.3. 5 changes fonts and decoration; 6 shifts the grid right. Build 4, 5 and 6 as one configurable 'Neutral' component family.
+
+## Implementation notes
+
+Built as `src/pdf/templates/neutral/` (one family component, `Layout.tsx`, with tokens in `linea.ts`; planning in `plan.ts`). Measured against `4.pdf` with `npm run templates:compare` (`/tmp/invoice-c3/compare-linea.png`, left reference, right ours).
+
+**Matches the design (measured, A4):** title baseline 161.6 pt and x 59.5 as in reference; hairline starting 20 pt after the title and ending on the margin; 'ISSUED TO' at x 77.4; the Montserrat glyph widths are metrically identical to the PDF (label and value widths agree to 0.1 pt); header baseline to first row 36.3 pt; row pitch 21.4 pt (reference 21.46); table rules at x 69.6-525.6 (reference 69.64-525.86); one colour, #3D3B3A.
+
+**Intentional deviations, and why:**
+- **Margins.** The template margin is 59.55 pt left and right, 56 top and 60 bottom, plus a 16 pt footer reserve. The doc's 77.4 pt text column with negative margins would put the title (x 59.5) and the rules (x 69.6) outside the margin box, which the audit forbids for flowing content. Here the title starts on the margin, the rules start 10.09 pt inside it and the text 17.9 pt inside it, so every x is relative to the margin and Narrow/Normal work. The content area ends at y 766 (page height less 60 + 16); the reference design's signature ink ends at 772.
+- **Supplier block (FROM)** is right-aligned under the title rule (docs: band x 290-535, y 170-255), max 245 pt wide. Its height pushes the info row down by the lines it needs beyond the design's empty band; a long address can never touch the title.
+- **PAY TO** stays under ISSUED TO as in the design, with Bank, Account Name, Account No., Branch and IFSC.
+- **Table.** An HSN/SAC column appears only when an item carries a code. UNIT PRICE shows the amount with a short unit ('₹1,200.00/hr'); when that does not fit the column it goes on two lines (amount, then 'per hour'). Fixed columns widen to their widest token (an amount cannot wrap) and the description takes the rest, so the table never stacks. The 4 pt indent of the cells (reference artefact) is dropped: text starts at x 77.4 in the header and the rows.
+- **Totals.** SUBTOTAL runs full width; DISCOUNT, TAXABLE VALUE, `CGST @ 9%` / `SGST @ 9%` (or UTGST, IGST) with their amounts and TOTAL form the right-anchored column (236 pt, amounts 92 pt). Amount in words and the reverse-charge line sit left of that column; below 150 pt of room they move under it.
+- **Closing.** Only the signature is bottom-right; the user's signature image or, without one, the signatory name and designation plus the computer-generated line. the reference design's script signature is not reproduced. The block is pushed to the foot of the last page by the bottom spacer.
+- **Logo.** None by default (so Linea matches `format.pdf`); `prefs.logo.show` (the Logo toggle next to the template button) puts the logo in the optional band above the title, at most 150 x 48 pt, bottom-aligned 4.85 pt above the title box.
+- **Elastic gaps** (Letter, and A4 when a one-page invoice overflows by less than the slack): the band above the title (68.85 to 33.85 pt), the supplier-to-info gap (down to 14 pt) and the info-to-table gap (49.5 to 24 pt). Type is never scaled. 'TAX INVOICE' drops its tracking to 0.12 em only if the hairline would be under 40 pt.
+- Footer ('Generated with Voiced', page n of N) at 7 pt Montserrat in the same ink; the design has none.